1. Field of the Invention
The present invention relates to lubricant compositions and to processes for producing such lubricant compositions. More specifically, the invention pertains to low viscosity lubricant compositions for the working surface of an extrusion die. The lubricants are particularly useful in the processing of plastics that require lubricants to render them processable, such as polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polyvinylidene chloride (PVDC) and copolymers thereof.
2. Description of the Related Art
For many years, plastics such as polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) and polyvinylidene chloride (PVDC) have been widely employed for various uses, including the fabrication of tubing and rigid pressurized piping. Of these materials, polyvinyl chloride is particularly desirable for forming rigid tubing and piping articles because of its low cost and beneficial properties, such as exceptional corrosion resistance to acids, corrosive liquids and gases. PVC tubing also exhibits smooth interior walls for unimpeded flow and reduced sediment buildup. It is non-contaminating, provides fast and reliable solvent-welded connections, offers good pressure-bearing capability, and provides ease of handling and installation. Although PVC is a very versatile polymer exhibiting such desirable properties, PVC base resin is also relatively hard and brittle and is very difficult to process with manufacturing equipment such as extruders, injection molders or other equipment used to process PVC compounds, such as prillers, flakers and pastillators. Accordingly, proper compounding and good lubricant balance are very important in obtaining good machine and end product properties.
It has been known in the art to use lubricant systems as extrusion aids to facilitate the processing of plastics such as PVC, CPVC and PVDC. Lubricants are materials that control the melting point in an extruder/molder to achieve the best processing characteristics and physical properties. Such lubricants may be external, internal or external/internal. External lubricants provide good release from metal surfaces and lubricate between the individual PVC particles and the metal surface. They are normally non-polar molecules, such as alkanes, and are usually paraffin waxes, mineral oils or polyethylene. External lubricants are normally incompatible with PVC, delay fusion and help the PVC slip over the hot melt surfaces of dies, barrels and screws without sticking and contribute to the gloss on the end product surface. They eventually migrate to the melt surface of the PVC, providing lubrication between the polymer and the metal surfaces of the extrusion equipment and are used primarily for processing of rigid PVC in applications where clarity is not a critical factor. External lubricants are prevailingly waxes, with the most conventional being paraffin waxes. Paraffin waxes are waxes that are separated from crude oil during the production of light lubricating oils. They are clear, odor free and can be refined for food contact.
Internal lubricants provide lubrication at the molecular level between resin particles and reduce the melt viscosity by improving inter-particulate flow when the PVC or other thermoplastic is in molten form. They are normally polar molecules, are usually fatty acids, fatty acid esters or metal esters of fatty acids and are very compatible with PVC. They lower melt viscosity, reduce internal friction and promote fusion. External/internal lubricants provide both external and internal lubrication depending on the combination of chemical groups contained. These have chemical groups of both lubricant types and generally have long hydrocarbon chains, along with amide, alcohol, acids and ester groups. Common types used in PVC are fatty acid amides and oxidized polyethylenes. Some of these materials will lubricate as an external lubricant before melting and as internal lubricants after melting. Others will do the reverse. Each of these lubricants should be characterized for its type of lubrication in a given PVC compound.
The purpose of lubrication is to achieve the full properties of a plastic and a melt that will extrude or mold without problems, at an economical cost. It has been found that highly effective lubricant compositions generally consist of a combination of at least one wax component, such as hydrocarbon waxes, alpha-olefin waxes and polyethylene waxes, and at least one fatty acid component or at least one metal salt of a fatty acid. However, it has also been found that known lubricant compositions blending waxes and such fatty acids or metal salts of fatty acids, undesirably increase the viscosity of the lubricant composition, rendering the lubricant difficult to process and finish on manufacturing equipment. Accordingly, a lower viscosity alternative is desired.
The present invention provides a solution to this need in the art. It has been unexpectedly found that the incorporation of an oxidized, low viscosity byproduct wax component reduces melt viscosity, allowing higher levels of a metal stearate to be incorporated, thereby producing an improved low viscosity lubricant composition. Particularly, the invention provides a multi-component lubricant composition comprising at least one oxidized, low viscosity wax and at least one metal salt, which metal salt comprises at least about 30% by weight of the lubricant composition. In the related art, U.S. Pat. No. 4,544,694, teaches a lubricant composition that is different than the present invention. In particular, their lubricant composition fails to include the oxidized, low viscosity byproduct wax component of the invention and they fail to teach a lubricant composition having the low viscosities taught herein.
The oxidized, low viscosity byproduct wax has a melt viscosity of less than or equal to about 100 centipoise at 140° C., an acid number of from about 7 to about 24 mg KOH/gram. Together with the metal salt component, the lubricant composition has a viscosity of less than about 450 cps at 140° C. Byproduct waxes from the polymerization of high density polyethylene are known, for example, from U.S. Pat. No. 5,728,754, which is incorporated herein by reference.
Incorporation of the oxidized, low viscosity wax into the blend permits levels of metal salt of about 70% or more while maintaining a melt viscosity low enough to process on conventional finishing equipment. Particularly, the lubricant composition can be used to provide internal and external lubrication when processing PVC compounds. Internally, it provides good melt blending and low processing temperatures, i.e. reduced shear heat, and externally it prevents the PVC compound from sticking to the metal surfaces of the processing equipment such as extruders, injection molders, prill towers, pastillating equipment, or other equipment used to process PVC compounds.